1. Field of the Invention
The present invention relates to a magnetic head for perpendicular recording and to a perpendicular magnetic recording and reproducing system.
2. Description of Related Art
In recent years, a hard disk drive which is a magnetic recording and reproducing system used as an external recording device for an information processor, such as a computer, has been required continuously to be increased in capacity and reduced in size. To meet the requirements, improvements have been made primarily in the recording density of the hard disk drive. If the density is increased by using a conventional longitudinal recording method, however, it is necessary to reduce the thickness of a recording layer since a demagnetizing field becomes maximum on the magnetic transition zone of a recording medium upon recording magnetization. This causes the problem that the resulting thermal disorder erases recorded data. By contrast, a perpendicular recording method in which the direction of recording magnetization coincides with the direction of the film thickness of the medium is more likely to achieve a higher recording density since the demagnetizing field becomes minimum on the magnetic transition zone and therefore it is less necessary to reduce the film thickness of the medium.
When a signal is recorded on a perpendicular recording medium by using a magnetic head for perpendicular recording, i.e., a perpendicular magnetic head, an electric signal is converted to a magnetic signal by a coil so that a magnetic flux is excited in a first magnetic pole or recording magnetic pole and a second magnetic pole or auxiliary magnetic pole. The magnetic flux forms a closed loop in which it extends from the second magnetic pole through the first magnetic pole, penetrates the perpendicular recording layer of the recording medium, passes through a soft seed underlying layer under the perpendicular recording layer, and returns to the second magnetic pole. The second magnetic pole is used to return, with magnetic efficiency to the first magnetic pole, a magnetic field formed in the first magnetic pole to extend to the perpendicular recording layer and soft under layer of the recording medium. By using the flow of such a magnetic flux, the signal is recorded on the perpendicular recording medium through magnetization. An example of the magnetic head is disclosed in Transactions of the Magnetics Society of Japan, Vol. 24, 335–338 (2000).
When the writing of data is performed in a magnetic recording system, three steps of erasing, writing, and collating are required. In a hard disk drive, however, a direct overwrite method in which writing is performed simultaneously with erasing is adopted to achieve a significant improvement in writing speed. An important issue in adopting the direct overwrite method is that a residual component of old data remaining after erasing is sufficiently small when new data is written directly over the old data. As an index indicative of the write performance of a magnetic head when data is overwritten, an overwrite erase ratio can be named, which is represented by a signal ratio before and after the overwriting of the old data.
When a magnetic head for perpendicular recording performs recording with respect to a magnetic recording medium in a magnetic disk apparatus, a tangent line to a recording position in the direction of rotation of the magnetic recording medium and a tangent line in the direction of film thickness of the first magnetic pole or recording magnetic pole of a magnetic head have a given angle (skew angle) therebetween. The skew angle differs depending on a radial position on the magnetic recording medium. When a magnetic head having a recording magnetic pole with a large film thickness performs writing with a skew angle being provided, a situation occurs in which a recording magnetic field is applied also to an adjacent track. If the occurrence of the situation is frequent, thermal demagnetization is accelerated disadvantageously in the recording magnetization of the adjacent track. A problem associated with such a magnetic head having a yaw angle has been reported in Transactions of the Magnetics Society of Japan, Vol. 25 591–594 (2001).
One approach to solving the problem is a reduction in the film thickness of the first magnetic pole or recording magnetic pole. As the film thickness of the recording magnetic pole is reduced in the magnetic head, however, the recording magnetic field is reduced in size and the distance between previous history magnetization on the leading edge side (leading side) and a magnetic transition point on the trailing edge side (trailing side) in the direction of travel of the magnetic head relative to the perpendicular magnetic recording medium is also reduced. As a result, interference from the previous history magnetization with a recording point shifts the magnetic transition point (transition point shift), which degrades the overwrite erase ratio. The overwrite erase ratio degraded by the transition shift is disclosed in, e.g., IEEE Transaction on Magnetics Vol. 30, No. 6, pp. 4272 (1994) “Relationship between Overwrite and Transition shift in Perpendicular Magnetic Recording”.
As shown in the report, in a perpendicular magnetic head and a perpendicular magnetic recording and reproducing system using the same, the prevention of the degraded overwrite property caused by a reduction in the film thickness of the first magnetic pole or recording magnetic pole of the magnetic head is an important issue.